In petrol-fed internal combustion engines, there is contemplated a butterfly valve which is arranged upstream of an intake manifold and adjusts the flow rate of the air which is fed to the cylinders. A known butterfly valve presents a valve body accommodating a valve seat engaged by a butterfly valve plate, which is keyed onto a rotational shaft to turn between an opening position and a closing position by effect of the action of an electrical motor coupled to the shaft itself by means of a geared drive.
A position sensor, which is adapted to detect the angular position of the shaft (i.e. of the butterfly valve plate), is coupled to one end of the shaft to allow a control unit to feedback-control the electrical motor. The electrical motor, the geared drive and the position sensor are accommodated within a valve body accommodation chamber, which accommodation chamber is closed by a removable lid.
In the valve body, there is a cylindrical tubular pipe, within which there is obtained an air introduction channel along which the valve seat is defined and the butterfly valve plate is thus arranged. A connection flange, which presents four holes which are crossed in use by corresponding fastening screws to rigidly fasten the valve body to the intake manifold, is provided on a first end of the cylindrical tubular pipe; instead, a flexible tube, which receives fresh air (i.e. air from the atmosphere) from an air vent provided with air cleaner and is fixed about the second end of the cylindrical tubular pipe by means of a tube clamp, is fitted about the second end of the cylindrical tubular pipe opposite to the first end.
It has recently been proposed to form the valve body by moulded plastic material instead of metallic material to reduce the manufacturing costs of the valve body itself. When the valve body is formed by moulded plastic material, the connection flange of the cylindrical tubular pipe is provided with four cylindrical reinforcement columns, each of which perpendicularly rises from the connection flange and is centrally perforated to accommodate a corresponding fastening screw. The function of these reinforcement columns is to locally increase the mechanical strength at the fastening screws and the presence of the reinforcement columns themselves is made necessary by the fact that the plastic material presents lower mechanical features with respect to the previously used metallic material.
In order to obtain an appropriate mechanical strength, each reinforcement column must be connected on top to the cylindrical tubular pipe of the valve body. However, it has been observed that at the connection point with the reinforcement columns, the cylindrical tubular pipe tends to presents deformations related to the phenomenon of “shrinkage” of the plastic material during the step of moulding. Such deformations of the cylindrical tubular pipe are particularly detrimental, because they alter the geometry of the air introduction channel in an area very close to (or even coinciding with) the valve seat concerned by the butterfly valve plate and thus determine an alteration of the butterfly valve performances.
In order to eliminate the presence of the reinforcements columns, it has been suggested to make a very thick connection flange (at least 15 mm); however, such constructive solution implies both a use of more material (and thus a higher cost and a heavier weight), and greater complications in the step of moulding of the valve body.